1. Field of the Invention
The present invention relates to an electrode, particularly a sintered type electrode, for alkaline storage batteries and a process for producing the electrode.
2. Description of the Related Art
While alkaline storage batteries have been enlarging their market with increasing use of portable communication equipment, office automation appliances, power tools and the like, batteries with higher capacity and more enhanced performances are eagerly desired.
Electrodes used for alkaline storage batteries are roughly divided into two types according to the method of production thereof. A sintered type electrode is obtained by impregnating, electrically or chemically, with an active material a substrate obtained by coating a core material with a paste formed by kneading carbonylnickel powder and a binder, followed by sintering. A paste type electrode is obtained by filling or coating a paste containing an active material into or on a metallic porous body, such as formed metal and nickel nonwoven fabric, or a two-dimensional core material, such as punched metal and expanded metal. The sintered type electrode, as compared with the paste type electrode, is featured in that though it is lower in capacity owing to the low porosity of the substrate, it has better charge-discharge characteristics at a large electric current.
In the sintered type substrate, also, attempts have been made to increase the porosity of the substrate in order to meet the demand for higher capacity. However, the increase of porosity causes the problems of lowering of adhesion of the core material to the porous body, deformation of the substrate in the steps of active material impregnation, chemical transformation, etc., and falling off of the nickel porous body or the impregnated or filled active material from the substrate.
With the intention of improving the adhesion of the core material to the porous body, JP-A-51-59348 proposed a process which comprises plating nickel on an iron plate at a current density of 5-20 A/dm.sup.2 to form coarse plating deposits on the iron plate surface, the adhering nickel powder to the core material obtained above, followed by sintering. JP-B-60-41426 proposed a process which comprises coating the core material with a slurry comprising a metal powder for sintering of an average particle diameter of 0.1 .mu.m or less and an adhesive, followed by drying and sintering, and then coating the surface of the sintered layer obtained above with a slurry comprising a metal powder for sintering of an average particle diameter of 1.5-4.5 .mu.m and an adhesive, followed by drying and sintering. JP-A-55-97403 disclosed a process which comprises preliminarily sintering a nickel fiber or nickel-plated metal fiber with a fiber diameter of 20 .mu.m or less on a core material and then sintering nickel powder thereon.
The above-mentioned processes intend to improve the adhesion of the porous body to the core material by increasing the contact area of the two. However, in all of the processes, the core material and the porous body are merely in point contact, and no substantial improvement in adhesion can be observed.